Coiled coil

Coiled Coil Structure: Ribbon Diagram of a Coiled Coil such as occurs in the fibrous proteins alpha-Keratin or tropomyosin.
 Fibrous proteins are, for the most part, characterized by highly repetitive simple sequences such as a coiled coil of two alpha helices such as occurs in alpha-keratin or tropomyosin. Shown here is a ribbon diagram of a coiled coil. One of the segment's two identical chains, "Coil 1", is yellow and the other, "Coil 2", is seagreen. The N- and C-terminal ends of each segment are marked by blue and red balls. You can readily see that the protein forms a parallel coiled coil: its component alpha helices are right-handed but the coiled coil is left-handed. Look at Views1-3 to see the coiled coil from different angles. Drag the 'zclip' slide left and right in View3 to see part or all of the structure. Note that the coiled coil makes only slightly more than 1/2 turn over a distance in which each alpha helix makes ~13 turns.

Drag the mouse from side to side horizontally across the image to see the way in which the two coils wrap around each other. Parallel coiled coils occur in many proteins including alpha keratin, a fibrous stress-bearing protein occurring in mammalian skin; tropomyosin and myosin, important proteins in muscle; and the so-called leucine zipper segments that permit the dimerization and hence activation of numerous eukaryotic transcription factors. Leucine zippers differ from other 2-helix coiled coils only in that their d-residues are almost invariably Leu.

The Heptad Repeat and Hydrophobic Interactions in Coiled Coils.
 Shown here is the structure of the coiled coil in atomic detail but showing only the Cb atoms of the sidechains so you can see the Heptad Repeat and Hydrophobic Interactions in Coiled Coils. View1 shows a centrally located 8-residue segment, "Piece-4" of each subunit, with the covalent bonds of Coil 1 yellowtint, those of Coil 2 greentint, and the Cb atoms represented by magenta balls. Click the "a-Residues" and "d-Residues" buttons to see the Cb atoms of the a and d residues highlighted in green and gold.

The amino acid sequence of the polypeptide exhibits a rough heptad repeat, (a-b-c-d-e-f-g)n, with the residues at the a and d positions having small hydrophobic side chains (Ala, Val, Leu, and Ile). This provides a hydrophobic strip on one face of each alpha helix that mediates its association with another such helix to form a coiled coil. The slight discrepancy between the 3.6 residues per turn of a normal alpha helix and the 3.5-residue repeat of the a- and d-residues causes this hydrophobic strip to wrap about its alpha helix in a gentle left-handed helix, thereby accounting for the formation of the left-handed coiled coil.

Go to View3 and turn on all seven "Piece-n" buttons to display the coiled coil in side view. Here, the protein is represented by its main chain and Cb atoms colored as described above. View3 through View8 show the coiled coil in various orientations about its superhelix axis. Go to View1 and drag the 'zclip' slide left and right to see part or all of the coiled coil viewed along its superhelix axis. Note how its a- and d-residue side chains are packed along the superhelix axis. The closeness of this packing accounts for the presence of only small hydrophobic side chains at these positions; larger hydrophobic side chains, those of Phe and Trp, in these positions would pry the two helices apart and thereby destabilize the coiled coil. Finally, click on several of the Cb atoms located about the periphery of the coiled coil to convince yourself that these residues are almost entirely hydrophilic in character.

Coordinates
The coordinates for the coiled-coil (tropomyosin) were obtained from Xiaoling Xia and Carolyn Cohen, Brandeis University.